1. Field of the Invention
The present invention relates to a tunable filter, and more particularly, to a method for tuning the tunable filter.
2. Description of the Prior Art
A tunable filter is a filter capable of having its frequency response tuned, and is widely used in various electronic devices because it provides a flexible filtering range.
Generally speaking, the frequency response of the tunable filter is decided by the status of its control signal. As the status of the control signal changes, the characteristic frequency of the tunable filter changes accordingly. The above-mentioned characteristic frequency is the corner frequency of the tunable low-pass (or high-pass) filter or the pass-band frequency of the tunable band-pass filter.
In many well-known tunable filters, the characteristic frequencies under various configurations are equally distributed within a probable frequency range. For example, FIG. 1 is a distribution diagram of the characteristic frequencies under various configurations of a prior art tunable low-pass filter. In this example, the corner frequency of the tunable low-pass filter is regarded as the characteristic frequency thereof, and the probable frequency range of the corner frequency is 4 MHz-40 MHz. As shown in FIG. 1, the characteristic frequencies under various configurations of the tunable low-pass filter are distributed in increments from 4 MHz to 40 MHz, with the distance between every adjacent characteristic frequency being 1 MHz.
However, the prior art method for distributing the characteristic frequencies in equal increments within the probable frequency range of the tunable filter has some drawbacks. For example, the control of the tunable filter becomes more complex, and at lower characteristic frequencies, the ratio between the step size of the characteristic frequency and the characteristic frequency of the tunable filter is too large, which worsens the signal-to-noise ratio (SNR) of the tunable filter.
Taking the characteristic frequency distribution diagram shown in FIG. 1 for example, because the characteristic frequencies under various configurations of the tunable low-pass filter are equally distributed from 4 MHz to 40 MHz, in which results in 37 probable characteristic frequencies, the control signal of the tunable low-pass filter requires at least 6 bits (i.e., the control signal has to be a 6-bit signal), and this increases the control complexity of the low-pass filter of the system. Additionally, when the characteristic frequency is equal to 4 MHZ, the next adjacent characteristic frequency is 5 MHz. The step size of the characteristic frequency of the tunable low-pass filter is 1 MHz, and the ratio between the step size and the characteristic frequency is 1 MHz/4 MHz=25%. If the data components of the input signal of the tunable low-pass filter are distributed from 0 MHz-4.1 MHz, it is necessary to tune the tunable low-pass filter to the status 02 in FIG. 1. That is, the corner frequency of the tunable low-pass filter has to be controlled at 5 MHz. However, it cannot filter out the noise in the input signal distributed from 4.1 MHz-5 MHz, which greatly degrades the signal-to-noise ratio of the tunable low-pass filter.